1. Field of the Invention
The present invention relates to a stress detection method for a force sensor device with a multiple axis sensor and a force sensor device employing this method.
2. Description of the Related Art
For varied control of mobile bodies such as automobiles and ships, robots and machine manufacturing equipment or mobile devices that are held and used by users such as notebook PCs, cellular phones, PDAs (Personal Digital Assistants), DVCs (Digital Video Cameras), DSCs (Digital Still Cameras) and so forth, for example, detection of the stress on all or part of a target according to the control objective is essential. Acceleration can be sensed by sensing the stress on the basis of the acceleration acting on all or part of the target.
A variety of devices for detecting this stress or acceleration has been developed conventionally. As one such device, a device that forms a gauge resistance on a semiconductor substrate and converts the resultant mechanical distortion on the basis of the force exerted from the outside into an electrical signal by means of the piezoresistive effect is known (“3-axis sensor development” ‘Inventions’ Journal, pages 52 to 63, September 2003; issued by the Invention society). In addition, the fact that changes in the electrostatic capacity corresponding to the distortion can also be utilized instead of using the piezoresistive effect of the gauge resistance is explained in “3-axis sensor development” ‘Inventions’ Journal, pages 52 to 63, September 2003; issued by the Invention society.
In particular, the invention described in “3-axis sensor development” ‘Inventions’ Journal, pages 52 to 63, September 2003; issued by the Invention society is characterized in that sensors of a plurality of types, that is, four types of sensor, namely a 3-axis force sensor, a 3-axis acceleration sensor, a 3-axis angular velocity sensor, and a 6-axis motion sensor can be constituted in one device by means of force sensors in a plurality of axis directions that are formed on the semiconductor substrate.
Further, when an acceleration sensor for sensing vehicle acceleration is used for vehicle brake control and so forth, a fault with the acceleration sensor causes unnecessary braking or an unstable state such as vehicle spin is judged. Therefore, a device for detecting acceleration sensor anomalies that serves to prevent such problems has been proposed (Japanese Patent Application Laid Open No. H9-113535). In the case of the invention of Japanese Patent Application Laid Open No. H9-113535, two acceleration sensors are each placed to form equal angles with an axis along front or rear direction of the vehicle and judging means that judge an anomaly with the one acceleration sensor, when a state in which the fluctuation width of the detected acceleration of either one of the acceleration sensors is less than a predetermined value continues for a predetermined time, and the fluctuation width of the detected angular velocity of the other acceleration sensor simultaneously exceeds a predetermined value.
In addition, as another invention, one technique that solves a problem where the direction of the detection axis in which the acceleration sensor detects and the direction of the physical quantity that is actually delivered to the sensor do not match has been proposed (Japanese Patent Application Laid Open No. 2003-227844).
The problem where the direction of the detection axis in which the acceleration sensor detects and the direction of the physical quantity that is actually delivered to the sensor do not match occurs in the following case, for example.
That is, in the case of a navigation device mounted in a vehicle, as the acceleration sensor finds the current position of the vehicle and, as a reference when detecting the displacement direction of the vehicle from a point of reference or a change in velocity, the sensor is equipped with a detection axis that is established in a fixed direction.
Thus, no problems occur when the direction of the detection axis matches the direction of the actual acceleration. However, generally, in cases where the navigation device is mounted in a vehicle, there are frequent cases where the navigation device is mounted tilted due to restrictions with the vehicle-mounting structure and the relationship with the interior and so forth. In this case, the direction of the detection axis and the direction of the actual acceleration do not match and the detected component in the direction perpendicular to the detection axis appears as noise. As a result, detection errors occur so that accurate positional measurement is difficult. Therefore, in order to deal with this problem, the mount angle in the vehicle is specified beforehand, and a built-in program corrects errors in the specified mount angle.
In view of the above facts, the invention that appears in Japanese Patent Application Laid Open No. 2003-227844 is constituted so that the sensor is attached at an angle in advance, in order that the specified mount angle at which the navigation device is mounted in the vehicle is cancelled, and the direction of the detection axis matches that of the actual acceleration, when the sensor equipped for a navigation device is mounted.
The 3-axis sensor development” ‘Inventions’ Journal, pages 52 to 63, September 2003; issued by the Invention society and the Japanese Patent Application Laid Open No. H9-113535 make no mention at all of problems arising from measurement errors caused by unnecessary outputs in the direction of the detection axis and the perpendicular direction. On the other hand, Japanese Patent Application Laid Open No. 2003-227844 discloses a method in which the mount angle of the sensor is pre-adjusted when a sensor is mounted on a sensor device in correspondence with the specified mount angle.
Therefore, in the case of Japanese Patent Application Laid Open No. 2003-227844, sensor devices with corresponding sensor mount angles must be prepared for each specified mount angle and therefore cannot be used generically for a variety of specified mount angles.